Multi-layer identity cards of plastic possess at least one card core of an integral card core layer or ply printed on both sides or at least two card core layers, which are in this case respectively printed on one side. An adhesive coated covering layer is then to be applied to each side of the card core printed on either side. The card layers are joined together in laminating process.
The adhesive coated covering layer, which is usually made transparent, has its adhesive coating adhering to the printed card core, and serves inter alia for protecting the print. Furthermore it is possible for the covering layer to contain certain additives for example, which are necessary for later laser inscription on the identity card.
In order to enhance the appeal of the identity cards to the eye or for safety reasons or for some other reason the cards are frequently printed on either side and not only on one side.
Such multi-layer identity cards are widely employed as check or bank cards. Frequently, such cards are also provided with a magnetic strip or an integrated circuit (semiconductor component or chip). Identity cards with a chip are known as chip cards or smartcards. Thus multi-layer identity cards with a chip are employed on a large scale as an authorization means for access to GSM mobile telephony so that when the owner of the identity card plugs it (the GSM card) in a mobile radio terminal he informs the mobile communications network that he is authorized.
For the card layers—both the card core layers and also the covering layers—various different materials come into question: polyvinyl chloride (PVC), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET, PETG and PETF). The selection of the materials and accordingly of the card structure is dependent on different factors. These factors are for instance: the desired mechanical properties of the identity card to be manufactured, its behavior at high and low temperatures, the question of printability, the question of laser inscription and furthermore economic and ecological determinants.
Dependent on which card structure is selected, different laminating parameters (temperature, pressure and time) come into play. Same are dependent more especially on which material is utilized for the card core layers.
Multi-layer identity cards are manufactured by firstly producing either the integral card core layer printed on both sides or producing the card core layers printed on one. Then the adhesive coated covering layers are produced. Following this the card layers produced are brought into register and in this state placed in a laminating press, wherein they are joined together under pressure and while heated. Moreover, there is a provision such that the card layers to be laminated are fed using rolls and are united in the so-called roll laminating method.
However in this case, more particularly in the case of fairly high temperatures of lamination (>120 degrees C.) a problem has occurred in the case of the lamination of identity cards with a card core printed on either side, which does not occur in the case of the lamination of cards printed on only one side.
In fact above a certain lamination temperature in the lamination of identity cards, which have a card core printed on either side (irrespectively of whether same is itself of single piece or of multi-layer structure), there is an undesired deformation or displacement of the printed card core in relation to the covering layers arranged on either side of it, which are in direct contact with the press platens acting on them during lamination. The printed card core and the covering layers are then no longer in register with each other and the outline of the card is no longer in accordance with requirements. The card must be discarded as a reject which can not be used.
The reason for the displacement of the printed card core in relation to the covering layers is taken to be that while the covering layers rest without any possibility of slip against the lamination platens on either side of the card structure to be laminated, above a certain lamination temperature the adhesive coating on the covering layers will form a sort of lubricating film between the printed card core and the covering layer, on or between which the printed card core moves out of position in relation to the covering layers. Then the formation of a lubricating film is even more encouraged owing to presence of the printing ink film.
This also seems to be the reason for this problem in the lamination of identity cards not occurring in cases wherein only one side of the card is printed. In this case the unprinted side of the card core—just like the opposite covering layer on the printed card side—is directly on the laminating press platen without any possibility of slipping and thus holds the card core layer in place, even if a lubricating film has formed on the other side between the covering layer and the printed card core. The problem also fails to occur in the case of cards which are not marginally printed.